The present invention relates to a thermistor having a positive temperature coefficient of resistance, and more particularly, it relates to a laminated type thermistor having a positive temperature coefficient of resistance.
An example of a prior art thermistor (hereinafter referred to as "PTC thermistor") having a positive temperature coefficient of resistance is formed of semiconductor ceramic material composed mainly of barium titanate and a small amount of a rare earth element such as niobium (Nb), antimony (Sb), tantalum (Ta), tungsten (W), yttrrium (Y) or another rare earth element. Manganese (Mn) is added as a characteristic improving agent for increasing the positive temperature coefficient of resistance, along with silicon dioxide (SiO.sub.2) and/or aluminum oxide (Al.sub.2 O.sub.3) serving as mineralizer.
Such a semiconductor ceramic device is generally formed with electrodes having ohmic properties. It is well known that such electrodes having ohmic properties may be formed of a metal such as indium-gallium alloy, nickel, or aluminum.
On the other hand, a practical PTC thermistor is required to be low in resistance. For example, it requires a low-resistance PTC thermistor having a resistance value of about 0.3 to 3.OMEGA. to protect the type of DC motor used in driving the power window of an automobile from overheating.
A circuit design for satisfying such requirement may include a plurality of PTC thermistors electrically connected in parallel with each other. However, such a prallel connection of PTC thermistors is not preferable since it substantially increases the size of the circuit.
Therefore, the present inventor has attempted to provide a laminated type of PTC thermistor by laminating ceramic layers together with a plurality of inner electrodes. Such a laminated type PTC thermistor comprises a monolithic ceramic sintered body which is obtained by firing a plurality of laminated ceramic layers and a pair of outer electrodes. The outer electrodes are formed on two different regions off the outer surface of the ceramic sintered body so as to connect each respective outer electrode to specific ones of the inner electrodes, whereby a plurality of resistors between respective pairs of inner electrodes are formed in parallel with each other.
The inventor attempted to employ a prior technique of manufacturing the aforementioned laminated type PTC thermistor, wherein the inner electrodes are formed of a metal having a high melting point such as gold (Au), platinum (Pt), palladium (Pd) or silver-palladium alloy, which is resistant to the high temperatures applied in a firing step included in the steps of manufacturing the PTC thermistor. More specifically, a paste containing such a metal having a high melting point may be coated by screen printing on ceramic green sheets, which in turn are laminated and then integrated by thermocompression bonding. The integrated body is then fired in an oxidizing atmosphere. However, such metals having high melting points cannot form ohmic electrodes, and accordingly are inappropriate for the inner electrodes of the laminated type PTC thermistor.